Anti-Müllerian hormone (AMH) [also called Müllerian inhibiting substance (MIS)] is a member of the transforming growth factor-beta family. AMH and its type II receptor (AMHR-II) are involved in the regression of the Müllerian ducts in the male embryo, and in gonadal functions in the adult. AMH is also known to be a marker of granulosa and Sertoli cell tumours. We selected a high-affinity monoclonal antibody, mAb 12G4, specific for human AMHR-II (hAMHR-II), by FACS analysis, Western blotting and immunohistochemical staining of a hAMHR-II-transfected CHO (Chinese hamster ovary) cell line, normal adult testicular tissue and granulosa cell tumours. Using peptide array screening, we identified the binding sequences of mAb 12G4 and AMH on the receptor. Identification of Asp53 and Ala55 as critical residues in the DRAQVEM minimal epitopic sequence of mAb 12G4 definitively accounted for the lack of cross-reactivity with the murine receptor, in which there is a glycine residue in place of an aspartic acid residue. In a structural model, the AMH-binding interface was mapped to the concave side of hAMHR-II, whereas the mAb 12G4-binding site was located on the convex side. mAb 12G4, the first mAb to be raised against hAMHR-II, therefore has unique properties that could make it a valuable tool for the immunotargeting of tumours expressing this receptor.